|I'm not sure at what point this discussion went off track completely, but a fair bit of the 'science' in here is abyssmal. Dude, simplest thing - climb more. Hanging on stuff isn't as simple and painless as people might make it look, get the techniques right and practise. Hopefully you're climbing with people more experienced who can give you pointers and demonstrate things. But you can learn stuff from anyone, because they all have different approaches to the same problems.
The pump! Everyones favourite excuse for falling off. Physiologically, caused mostly by a buildup of fluid in the tissues / muscooles. More fluid in muscles = they get big = PUMPED! Physical effort = increase in heart rate. Contraction of muscles blocks off capillaries - buildup of blood pressure at heart side of capillary, causes fluid to leave capillary. Extracellular fluid builds up within the muscle, doesn't drain back in to capillaries and venules anywhere near as quickly because the pressure gradients dont favour it. This fluid returns, as people have said, via the lymphatic system. This system relies in part on gravity, but also on skeletal muscle contractions. Gravity aint gunna push the fluid up your arms if they're by your sides, so hang with them above your head. Try NOT to have muscles contracted while they're up there, coz that'll block most of the flow, but if you can slowly twitch them, that will force fluid along the lymphatics.
More training of muscles = more capillaries = more O2 to muscles. Also = more mitochondria = more utilisation of O2 to produce ATP (cellular energy from glucose) without production of lactic acid.
Kidneys provide much of the pH regulation within the body, Phil S. However, many other systems are involved, including buffer molecules and the lungs. Lactic acid -> H+ ions -> cause more delivery of O2 from haemoglobin. Is good, assuming can get blood and O2 to muscles to start with.
CO2 is removed entirely by the blood. Lactic acid removal i'm less sure about, but blood flow would definitely be important. Fresh blood good. Spicelab, being fitter results in your heart being able to pump more blood with each contraction, so slower rate = same output.
I think thats everything. Probably not, but anyway. Gordoste, if you can track down one of those brain pumps, i could really use it. Got exams coming up in a coupla weeks...